If anything the additional stacks means both app quality and the platform-specific knowledge a senior dev needs has declined, since today you're probably working on a pile of React Native bullshit or something that has very little to do with Android anyway.

True in the case of React Native bullshit, false in the case of a native app that isn't just CRUD. A senior without android experience could jump on the apps that are glorified websites and be productive relatively quickly.

A senior without android experience is going to have to learn the platform; this will take longer than learning the language. The pace of change updates the best practices often enough to serve as a kind of catch-up mechanic but native mobile does in fact require domain knowledge. I would be comfortable putting a senior iOS dev on native android but certainly not as the lead.